This invention relates to a process for the partial oxidation of hydrocarbons to produce gaseous mixtures comprising hydrogen and carbon monoxide, such as synthesis gas, and fuel or reducing gas.
In particular, this invention relates to a partial oxidation process which comprises the steps of:
feeding a hydrocarbon-comprising gas flow into a reaction chamber;
feeding a free oxygen-comprising gas flow into said reaction chamber.
Throughout this specification and the appended claims, the term: xe2x80x9chydrocarbon(s)xe2x80x9d, is used to denote a light and/or heavy saturated and/or unsaturated hydrocarbon or hydrocarbon mixtures (e.g. C1-C6); the expression xe2x80x9chydrocarbon-comprising gas flowxe2x80x9d is used to either denote a fluid which contains gaseous hydrocarbons, such as methane or natural gas, or a gaseous flow comprising suspended solid combustible (e.g., coal dust or carbon soot), or a gaseous flow comprising dispersed liquid hydrocarbons (e.g., such light or heavy hydrocarbons as naphtha or fuel oils).
In technical language, a gas flow which contains suspended liquid hydrocarbons is usually referred to as a xe2x80x9cmistxe2x80x9d, while a gas flow which contains dispersed solid hydrocarbons is termed a xe2x80x9csmokexe2x80x9d.
The invention also concerns a burner for implementing the above process.
As is known, in the field of hydrocarbon partial oxidation there exists a pressing demand for a high yield process which can be easily implemented, and is both energy and cost efficient.
To fill the above demand, processes have been developed wherein the oxidation reaction is carried out at relatively low temperatures, on the order of 1300xc2x0 C., to significantly reduce oxygen consumption and produce hydrogen and carbon monoxide more economically.
A process of this kind is described in EP-A-0 276 538, for example, wherein a hydrocarbon-comprising gas flow is first mixed with a recovered solution comprising carbon soot and then, following evaporation of the water contained in the solution, mixed with oxygen in a reaction chamber at a temperature in the 927xc2x0 to 1316xc2x0 C. range, the combustion to hydrogen and carbon monoxide taking place in that chamber.
While this prior process does afford a reduction in the energy consumption in the reaction chamber, as well as in the amount of oxygen to be fed into the reaction chamber, it has a number of disadvantages, as listed herein below.
First of all, the carbon soot formed from the hydrocarbons pyrolysed in the reaction chamber which, in the proximity of the burner, get in contact with and are admixed to the hot gases circulating within the chamber before they can be suitably mixed with oxygen.
This production of carbon soot is mainly disadvantageous in that a whole series of energy-intensive operations are made necessary for separating the carbon soot from the reaction products and feeding it back into the reaction chamber, that a more complicated plant is needed for implementing the process, and that capital and operating cost is high.
In addition, the carbon soot produced inside the reaction chamber affects the overall yield of the partial oxidation process, lowering the amount of hydrogen and carbon monoxide which can be obtained per unit of burned hydrocarbon, even where all the carbon soot produced and returned to the burner is gasified.
On the other hand, prior processes effective to produce low carbon soot concentrations involve operating the reaction chamber at very high temperatures (on the order of 1400xc2x0 C.), and therefore, at a high rate of oxygen consumption and low conversion rate, for example as described in EP-A-0 276 538, page 2, lines 6-13.
In addition, the plants for implementing the aforementioned processes have a disadvantage in that they are inflexible in operation, being unable to accommodate the large load variations to which the reactants fed into the reaction chamber can be subjected, with the result that the variations may trigger or boost the formation of carbon soot.
It is on account of such limitations that prior art processes for the partial oxidation of hydrocarbons have involved large investment costs for their practical implementation, thereby significantly penalizing the production costs of such basic materials as hydrogen and carbon monoxide, and this in the face of a growing demand for them. Moreover, a pressing demand in the field for hydrocarbon waste matter as the residues from distillation processes in the oil industry to be burned off cannot be satisfactorily filled by the aforementioned prior processes.
The underlying technical problem of this invention is to provide an improved process for the partial oxidation of hydrocarbons, at high yield, which allows a high production of hydrogen and carbon monoxide per unit of burned hydrocarbon, while drastically lowering the formation of carbon soot even when operating at low temperatures, and is flexible and easy to implement for a reasonably low energy consumption and operating cost.
According to the present invention, the above problem is solved by a process as indicated above, which is characterized in that it further comprises the steps of:
mixing and reacting a first portion of said free oxygen-comprising gas flow with a first flow comprising reacted gases circulating within said reaction chamber;
mixing a second portion of said free oxygen-comprising gas flow with said hydrocarbon-comprising gas flow in said reaction chamber, obtaining a gas flow comprising both hydrocarbons and free oxygen at least partly mixed together;
mixing and reacting said gas flow comprising both hydrocarbons and free oxygen at least partly mixed together with a second flow comprising reacted gases circulating inside said reaction chamber, obtaining a gas flow comprising hydrogen and carbon monoxide.
Throughout this specification and the appended claims, the expression: xe2x80x9cgas flow comprising reacted gasesxe2x80x9d, is used to denote a gas flow which contains H2O, CO2, trace hydrocarbons, H2S, COS, and possibly N2 and Ar circulating inside the reaction chamber, additionally to the partial combustion products, i.e. CO and H2.
Advantageously, this invention enables the production of hydrogen and carbon monoxide per unit of burned hydrocarbon to be increased substantially relative to the prior art processes.
In fact, thanks to the step of mixing a portion of the free oxygen-comprising gas flow with the hydrocarbon-comprising gas flow within the reaction chamber, before the last-mentioned flow contacts the hot gases circulating inside the chamber, the formation of carbon soot during the following combustion step can be prevented or at least reduced drastically.
In this way, the conversion yield of the hydrocarbons in the reaction chamber will be only marginallyxe2x80x94if not at allxe2x80x94affected by the presence of carbon soot, thereby ensuring an optimum production in hydrogen and carbon monoxide.
It should be noted that thanks to the present invention the formation of carbon soot in the reaction chamber can be totally suppressed when the flow being processed comprises gaseous hydrocarbons, and can be held down to a bare minimum even where the gas flow comprises liquid and/or solid hydrocarbons.
This result is advantageously obtainable even when operating at low temperatures, preferably in the 950xc2x0 to 1300xc2x0 C. range, and therefore, at a lower rate of oxygen consumption and higher yield (increased production in CO and H2) than the prior art.
As an example, for the partial oxidation of natural gasxe2x80x94in a condition of total absence of carbon sootxe2x80x94the oxygen requirement can be kept lower than 210 moles O2 per kilomole of dry gas produced, which represents quite a surprising achievement compared to the requirements for oxygen of prior art processes.
In other words, the process of this invention prevents a portion of the hydrocarbons flowing through the reaction chamber from becoming mixed, in the absence of oxygen, directly with the high-temperature (e.g., in the 1000xc2x0 to 1400xc2x0 C. range) gases circulating within the chamber, causing the hydrocarbons to be pyrolysed and carbon soot formed. On the contrary, inside the reaction chamber, the hydrocarbons are first suitably mixed with the free oxygen, and only later contacted with the hot gases, which gases will then trigger an advantageous combustion, rather than pyrolysis, reaction of the reactants at least partially pre-mixed, to produce hydrogen and carbon monoxide.
Furthermore, the process of this invention is quite simple, economical and easy to implement, and involves neither a high energy consumption nor high operating and maintenance costs.
It should be noted that for the combustion of gaseous hydrocarbons, such as methane or natural gas, the plant implementing this process requires no carbon soot separation and re-circulation section, thereby affording major savings in investment cost and energy consumption over prior art plants.
Advantageously, the present process has proved highly flexible, since it can accommodate a range of different operating conditions while retaining its high conversion yield.
In particular, this process can be effectively applied even in case of large variations in the rate of the flows fed to the reaction chamber, such as in the 0.2 to 1.0 range (ratio of minimum to maximum flow rate), without affecting the conversion yield, a feature this one that cannot be found in the prior art processes.
The portion of the free oxygen-comprising gas flow which gets mixed, inside the reaction chamber, with the hydrocarbon-comprising gas flow before contacting the re-circulated reacted gases, referred to as the second portion in the process according to the invention, advantageously comprises as from 10 to 90%, preferably 50 to 70%, of the free oxygen-comprising gas flow.
In a particularly advantageous embodiment of the invention, this process comprises the step of feeding the hydrocarbon-comprising gas flow and the free oxygen-comprising gas flow into the reaction chamber as respective, substantially annular jets coaxial with each other.
Thus, the mixing of the hydrocarbons and free oxygen can take place in a most effective and prompt manner inside the reaction chamber.
Moreover, it has been found that to promote the mixing action, it is more advantageous if the hydrocarbon-comprising gas flow is fed to the reaction chamber outwardly of and preferably at a higher velocity than the free oxygen-comprising gas flow.
Preferably, according to the above embodiment, the process of this invention further comprises the steps of:
causing said free oxygen-comprising gas flow to flow through a first, substantially cylindrical conduit of predetermined length of a burner extending into said reaction chamber;
causing said hydrocarbon-comprising gas flow to flow through a substantially annular free space defined between said first conduit and a second outer conduit coaxial with the first, said second conduit being longer than said first conduit and defining inside said reaction chamberxe2x80x94between one end of said second conduit and one end of said first conduitxe2x80x94a mixing zone for said hydrocarbon-comprising gas flow and said free oxygen-comprising gas flow;
directing said hydrocarbon-comprising gas flow from said substantially annular free space to a region of said mixing zone close to an inner wall of said second conduit;
expanding and directing said free oxygen-comprising gas flow exiting said first conduit toward said inner wall of said second conduit in said mixing zone, thereby to mix and react a first portion of said free oxygen-comprising gas flow with a first flow comprising reacted gases circulating inside said reaction chamber in a central zone thereof, and to mix a second portion of said free oxygen-comprising gas flow with said hydrocarbon-comprising gas flow obtaining a gas flow comprising both hydrocarbons and free oxygen at least partly mixed together.
In this way, a desired pre-mixing of the hydrocarbons and the free oxygen can be achieved in the reaction chamber in a highly effective and reliable manner, while preventing during this step all contact of the hydrocarbons with the reacted gases being circulated within the chamber.
Advantageously, this pre-mixing is made to occur at a part of the inner wall of the feed conduit for the hydrocarbon-comprising gas flow which extends between its end and the end of the feed conduit for the free oxygen-comprising gas flow.
In practice, part of the free oxygen-comprising flow is advantageously caused to enter the hydrocarbon-comprising flow, and a sufficient degree of mixing is attained in a very small space to preventxe2x80x94in case of gaseous hydrocarbonsxe2x80x94or drastically reducexe2x80x94in case of liquid and/or solid hydrocarbonsxe2x80x94the formation of carbon soot during the subsequent admixture to hot gases circulating inside the reaction chamber.
In order to promote the expansion and transport of the free oxygen-comprising gas flow toward the inner wall of the second conduit in the mixing zone, this gas flow is preferably caused to flow through the first conduit along a spiral flowpath.
According to a further aspect of the invention, a burner for the partial oxidation of hydrocarbons is provided which comprises:
a first, substantially cylindrical conduit of predetermined length which defines on its interior a circular passageway for feeding a free oxygen-comprising gas flow into a reaction chamber outside the burner;
a second conduit, outer of and coaxial with but longer than the first, which defines a substantially annular free space on its interior between said conduits, for feeding a hydrocarbon-comprising gas flow into said reaction chamber;
and is characterized in that it further comprises:
a mixing zone, wherein said hydrocarbon-comprising gas flow is mixed with said free oxygen-comprising gas flow, defined between respective ends of said first and second conduit;
means for directing said hydrocarbon-comprising gas flow from said substantially annular free space to a region of said mixing zone close to an inner wall of said second conduit;
means for expanding and directing said free oxygen-comprising gas flow exiting said first conduit toward said inner wall of said second conduit in said mixing zone, thereby to mix and react a first portion of said free oxygen-comprising gas flow with a first flow comprising reacted gases circulating within said reaction chamber in a central zone thereof, and to mix a second portion of said free oxygen-comprising gas flow with said hydrocarbon-comprising gas flow obtaining a gas flow comprising both hydrocarbons and free oxygen at least partly mixed together.